The present invention relates to communications receivers and, more particularly, to a communications receiver having adaptive dynamic range in the presence of time-varying DC voltage drift.
In a typical communication system, analog signals are transmitted from a transmitter to a receiver through a transmission channel. A typical receiver includes an amplifier, an analog-to-digital (A/D) converter and an automatic gain controller. The amplifier amplifies the received AC analog signal, which is then converted by the A/D converter into a series of digital values that can be processed. The automatic gain controller monitors the signal power and adjusts the amplifier gain to restore the received signal to desired power level.
The A/D converter has a maximum peak-to-peak voltage range within which it is capable of converting analog voltage levels into corresponding unique digital values. Analog voltage levels outside that range are xe2x80x9cclippedxe2x80x9d and converted into a digital value corresponding to the next closest analog voltage level within the range. The size of the range is determined by the number of bits in the A/D converter and the resolution of each bit. These values are typically tuned for a particular application so that the dynamic range of the A/D converter matches or slightly exceeds the expected peak-to-peak voltage span of the received AC analog signal.
A common difficulty encountered in many communications receivers is DC voltage drift in the peak-to-peak voltage span of the received AC analog signal. DC voltage drift can cause the AC analog signal to shift outside the dynamic range of the A/D converter, which results in clipping of the analog signal and consequent loss of data. DC voltage drift is common in a variety of communications systems, such as systems that receive low frequency random data signals, systems that use transformer coupling, AC coupling and DC notching and any other system suffering from DC voltage drift or baseline wander at the transmitter or receiver or in the transmission channel. Examples of such systems include wire line modems, digital subscriber line (DSL) and cable communication systems, and Ethernet, radar and television receivers.
There are two common methods of correcting for DC voltage drift. The first is to allow for sufficient dynamic range or xe2x80x9chead roomxe2x80x9d in the A/D converter to ensure operation without clipping. This can be done by either increasing the number of bits in the A/D converter or by reducing the resolution of the converter to cover a larger peak-to-peak voltage span with the same number of bits. However, the cost and complexity of the A/D converter increase exponentially with the number of bits in the converter. Also, decreasing the resolution decreases the signal-to-quantization noise ratio of the A/D converter and hence degrades the performance of the receiver.
The second common method of dealing with DC voltage drift is to correct the drift prior to the input of the A/D converter. This option requires a good estimate of the DC voltage drift, which is preferably performed digitally. This requires additional circuitry, such as a digital-to-analog (D/A) converter or a delta modulator, to transport the DC drift estimate back to the analog domain such that the DC drift can be corrected prior to the input of the A/D converter. The addition of such a mixed signal device adds to the complexity of the hardware, which makes the design and testing of the hardware more difficult. Moreover, if the DC drift varies rapidly with time, fast estimation and correction of the DC drift becomes critical, which makes the use of more economic methods of under-sampled D/A converters or delta modulators prohibitive.
Improved methods and circuits for expanding the effective dynamic range of A/D converters in communications receivers are desired.
One aspect of the present invention relates to a communications receiver, which includes an analog input for receiving an analog signal having a time-varying DC voltage drift. A variable gain amplifier is coupled to the analog input and is adapted to amplify the analog signal based on a gain set by a gain control input to the amplifier. An analog-to-digital (A/D) converter is coupled to an output of the amplifier and is adapted to convert the amplified analog signal to a series of digital values. A drift estimator is coupled to the A/D converter, which generates an estimate of the time-varying DC voltage drift based on the series of digital values. A gain adjuster is coupled between the drift estimator and the amplifier, which adjusts the gain control input based on the drift estimate.
Another aspect of the present invention relates to a method expanding the effective dynamic range of a communications receiver, which includes a variable gain amplifier and an analog-to-digital (A/D) converter. The receiver receives an analog signal having a time-varying DC voltage drift. The analog signal is amplified with the amplifier to generate an amplified analog signal. The gain of the amplifier is set to a nominal gain level. The amplified analog signal is converted to a series of digital values by the A/D converter. The DC voltage drift is estimated based on the series of digital values. The gain is reduced from the nominal gain level to a reduced gain level during a time window in which the estimated DC voltage drift exceeds a threshold value.
Yet another aspect of the present invention relates to a communications receiver for receiving an analog signal having a time-varying DC voltage drift. The receiver includes an amplifier for amplifying the analog signal. The gain of the amplifier is set to a nominal gain level. An analog-to-digital converter converts the amplified analog signal to a series of digital values. A drift estimator estimates the DC voltage drift based on the series of digital values. A gain adjuster reduces the gain from the nominal gain level to a reduced gain level during a time window in which the estimated DC voltage drift exceeds a threshold value.